Horizontal deflection circuit for television systems

ABSTRACT

To protect the horizontal deflection circuit against reverse polarity high-voltage pulses derived from an inductance, which is provided to rapidly drain stored charge carriers of an output transistor thereof, a protective circuit is connected between the driver stage and the base of the output transistor which includes a transistor controlled by the driver stage, for example by a reverse polarity signal which renders the parallel connected transistor conductive, thereby short-circuiting the reverse polarity pulse with respect to the driver stage and protecting the driver stage thereagainst.

The present invention relates to a horizontal deflection circuit fortelevision systems, and more particularly to such a circuit in which adriver stage is directly coupled to an output transistor.

BACKGROUND AND PRIOR ART

Horizontal deflection circuits which are directly coupled have beenproposed in which the final or output power transistor has a voltage ofreverse polarity applied thereto so that it will turn OFF rapidly. Thisvoltage usually is comparatively high with respect to the voltage beingswitched by the output transistor. The reason for applying a highreverse voltage to the transistor is this: The charge carriers which arestored in the base of the output transistor are thus rapidly drained andthe losses in the final output transistor of the horizontal deflectioncircuit are thus substantially decreased.

Horizontal deflection circuits in which the final or output powertransistor is directly driven from a driver stage have the disadvantage,however, that the high reverse voltage applied to the output transistorcan damage and possibly destroy the driver stage.

The Invention

It is an object to protect the driver stage of a horizontal deflectioncircuit of the type in which the output transistor receives a reversecharge carrier drainage voltage, which is reliable, and operatesessentially trouble-free.

Briefly, a separate protective circuit is provided which is operatedduring the blocking phase of the final output transistor and whicheffectively electrically separates the driver stage from the outputtransistor. In a preferred form, the protective circuit includes atransistor controlled by the driver stage itself which preventsapplication of the reverse polarity voltage pulse to the driver stage.In accordance with a particularly suitable arrangement, the driver stageis protected by short-circuiting the signal path from the driver stageto the final output transistor during the blocking phase of the outputpower transistor. Short-circuiting of the driver stage may be controlledby signals derived from the driver stage itself. In a preferred from, atransistor is used which is so controlled from the driver stage that itbecomes conductive during application of the reverse voltage pulse tothe final output stage, thereby short-circuiting the signal path to thedriver stage.

Drawing, illustrating a preferred example, wherein the single FIGURE isa general, highly simplified block circuit diagram of the output portionof a horizontal deflection circuit for a television receiver.

A line or horizontal oscillator 5 is connected to a driver stage 1 tocontrol the driver stage 1 with horizontal frequency pulses. The driverstage 1 controls conduction or cut-off of output transistor T₂. Thecollector portion of the output transistor T₂ is connected to thehorizontal deflection system 4 of the TV image tube. A positive voltageis applied at terminal 6 to the horizontal deflection system, the otherterminal of the positive voltage being connected to ground or chassis.The system 4 may be of any well known and suitable type and includes--asstandard in the art--the horizontal deflection coils. associatedcorrection elements and the like and, if desired by the designer, theprimary coil of a horizontal output transformer. The horizontaloscillator 5 and the driver stage 1, itself, may be built in accordancewith any well known or desired circuit.

The output transistor T₂ is an npn transistor. Pulses from the driverstage 1 are conducted to the base of the transistor T₂ over diode D andcapacitor C. The diode is poled in conductive direction for the controlpulses. A coupling resistor R is serially included in the circuitbetween the driver stage and the base of the transistor T₂. The voltageapplied to the output transistor T₂ is also connected to an inductancecoil L, in which current increases with increasing power applied to thebase of transistor T₂. The blocking phase of the output transistor T₂ isinitiated by applying a voltage from the driver stage 1 to the base ofpnp transistor T₁ to render the transistor T₁ conductive. This voltageis effective during the OFF time of the output transistor T₂. The signalpath between the protective circuit and the output transistor T₂ is thusshort-circuited over the then conductive emitter-collector mainswitching path of the transistor T₁. The voltage which occurs at theoutput transistor T₂ and which would have held it conductive is then nolonger present. A magnetic field which is stored in the inductance Lrapidly collapses, providing a high reverse voltage pulse, in this casenegative, to the base of the output transistor T₂. This reverse voltage,high voltage voltage pulse drains the charge carriers which are storedin the base of the output transistor T₂. The high reverse voltage whichoccurs in this initial OFF phase of the output transistor T₂ is alsocoupled through the resistor R and the capacitor C to the now conductivecollector-emitter path of the transistor T₁. The current is limited bythe resistor R. The driver stage 1 is thus isolated from voltage peaksdue to the inductance L by the then conductive transistor T₁ ; theemitter-collector path of the transistor T₁ short-circuits such voltagepeaks with respect to the driver stage 1, thus protecting the driverstage 1 against these peaks.

The combination formed by the resistor R and capacitor C forms a timeconstant circuit and is an additional protection; the time constant ofthe R/C circuit will delay the time at which the voltage peak due to theinductance L is applied to the transistor T₁. This ensures that thetransistor T₁ is conductive when the voltage peak from the inductance Lis applied thereto.

The capacitor C also effects a voltage shift so that the base of theoutput transistor T₂ can have a negative bias applied thereto in asuitable manner without loading the driver stage. Such a negative biassupports drainage of charge carriers from the base zone of the outputtransistor T₂ during its OFF or blocked phase.

The protective circuit 2 may be constructed in various ways, and thediode D can be replaced by a resistor over which the pulses which switchthe output transistor T₂ are applied from the driver stage 1 to the basethereof.

The pulses which control the output power transistor T₂ and thetransistor T₁ during the OFF or blocked phase of the power transistor T₂can be connected in various ways; for example, the driver stage may haveseparate complementary outputs which are connected, respectively, to thebase of the output transistor T₂ and to the base of the protectivecircuit transistor T₁, respectively.

The horizontal deflection circuit, protected against overvoltage, issimple and has high efficiency and reliability. The protective circuitand the driver stage can be constructed easily by using suitableintegrated circuit technology.

The emitter-base diode of the protective transistor T₁ is in the signalpath of the driver stage to the output power transistor T₂, thecollector being grounded, as seen in the figure. The emitter-base diodeof transistor T₁ is reversely polarized with respect to the diode D, sothat the output pulses from the driver stage 1 which control transistorT₂ to be ON are conducted over the diode D, whereas the emitter-basediode of the protective transistor T₁ is rendered conductive by reversepolarity output pulses derived from the driver stage during the OFF orblocking phase of the output transistor T₂. The presence of theemitter-base diode portion of the transistor T₁ also permits thesimplification of the circuit by replacing the diode D by a resistor,which then will bridge the emitter-base diode of the transistor T₁.

It is a specific advantage of the arrangement that the high reversevoltage which builds up during the OFF or blocking phase of the outputtransistor T₂ can be removed from application to the driver stagewithout substantial additional materials or costs. The signal path fromthe driver stage to the output transistor has the resistor R includedtherein across which the voltage will build up which controls the outputtransistor T₂ to conduction during the ON phase of the output stagewhile permitting application of a high reverse voltage pulse to the baseof the transistor T₂ by connecting the inductance L between the resistorR and the base of the transistor T₂. The resistor thus has the multiplefunction of current limiting for the transistor T₁, forming a portion ofthe R/C time delay circuit which protects the driver stage whentransistor T₁ is conductive and additionally limits the current flowthrough the emitter-collector main current carrying path of thetransistor T₁. The protective circuit 2 can be readily integrated withthe driver stage 1 in a single integrated circuit without substantialadditional cost, and permits constructing the driver stage with elementsof low voltage rating, thus decreasing the cost of the overall circuit,while providing reliable protection against reverse voltage peaks to thedriver stage and still effecting rapid turn-off of the power outputtransistor T₂.

Various changes and modifications may be made and, particularly, thediode D can be replaced by a resistor.

In a typical television horizontal deflection circuit for a 27" diagonalTV tube, the inductance L is: 1 mH

resistor R: 22 Ohms

capacitor C: 2.2 μF resulting in a reverse voltage pulse at the base oftransistor T₂ of ≦5 volts which can readily be isolated from the driverstage 1 by an integrated circuit transistor T₁ of 1.8 Aemitter-collector current carrying capacity.

I claim:
 1. Horizontal deflection circuit for a television system havingan output transistor (T₂);a driver stage (1) connected to the base ofthe output transistor (T₂) to alternately control the transistor toconduction and blocking state; means (L) providing a high-voltage pulseof reverse polarity to the base of the output transistor (T₂) to causerapid change-over from conduction to blocking state thereof, and drainstored charge carriers, and comprising, in accordance with theinvention, a protective circuit (2) to protect the driver stage againstthe reverse high-voltage pulse including controlled switch means (T₁)controlled by the driver stage (1) and preventing application of saidreverse polarity high-voltage pulse to the driver stage.
 2. Circuitaccording to claim 1, wherein said controlled switch means (T₁)comprises a controlled semiconductor switch which is connected toshort-circuit the signal path (3) from the driver stage (1) to theoutput transistor (T₂) during the time that the output transistor is inblocking state to thereby prevent application of said high-voltage pulseto the driver stage.
 3. Circuit according to claim 2, wherein thecontrolled switch means comprises a transistor (T₁) and connection meansfrom the driver stage (1) to the base of said transistor to render saidtransistor conductive and hence short-circuit the signal path (3) fromthe driver stage (1) to the output transistor (T₂).
 4. Circuit accordingto claim 3, wherein the controlled switch means transistor (T₁) has itsemitter-base diode connected to the signal path (3) between the driverstage (1) and the base of the output transistor (T₂);the collector ofthe controlled switch means transistor (1) being connected to referenceor ground potential; and a diode (D) is connected in parallel to saidemitter-base diode and reversely poled with respect thereto, thepolarity of said diode transferring the output signals from the driverstage (1) to the output transistor (T₂) in conductive direction, and thedriver stage (1) provides output pulses to the controlled switch meanstransistor (T₁) of a polarity reverse to that of the signals controllingthe output transistor to render said controlled switch means transistor(T₁) conductive during the blocking phase of the output transistor (T₂).5. Circuit according to claim 3, wherein the controlled switch meanstransistor (T₁) has its emitter-base diode connected in the signal pathfrom the driver stage (1) to the output transistor (T₂);a resistancemeans (D) is connected in parallel to said emitter-base diode; thecollector of the controlled switch means transistor (T₁) being connectedto ground or reference voltage; and the driver stage (1) provides outputsignals of respectively reverse polarity, the output signals of onepolarity being transferred from the driver stage (1) through saidresistance means (D) to the base of the output transistor (T₂) andhaving a polarity which will block the emitter-base diode of saidcontrolled switch means transistor and, during the blocking phase of theoutput transistor, the driver stage provides output signals of apolarity which will render the emitter-base diode of said controlledswitch means transistor (T₁) conductive to thereby render saidcontrolled switch means transistor (T₁) conductive and preventapplication of the high-voltage reverse polarity pulse from saidhigh-voltage pulse providing means (L) to the driver stage.
 6. Circuitaccording to claim 3, further including a resistor (R) in the signalpath (3) from the driver stage (1) to the output transistor (T₂) andconnected between the protective circuit (2) and the outputtransistor;and said means (L) providing the high-voltage pulse comprisesan inductance (L) connected to the resistor at a terminal remote fromthe protective circuit (2).
 7. Circuit according to claim 6, furtherincluding a capacitor (C) connected in circuit with the resistor andproviding, together with the resistor, an R/C time delay circuit todelay application of the reverse polarity high-voltage pulse to theemitter-collector circuit of said controlled switch means transistor(T₁) to ensure conduction of said controlled switch means transistor(T₁) before application of said reverse polarity pulse thereto. 8.Circuit according to claim 1, further including a time delay circuit(R/C) connected between said means (L) providing the high-voltage pulseand the protective circuit (2) to ensure protective operation of saidprotective circuit in advance of application of said high-voltagereverse polarity pulse thereto.
 9. Circuit according to claim 2, furtherincluding an R/C time delay circuit between the means (L) providing thehigh-voltage pulse and said controlled switch means (T₁) to ensureprotective short-circuiting operation of said controlled switching meansin advance of the application of the reverse polarity high-voltage pulsetherethrough.